Tissue regeneration during lower jaw restoration in zebrafish shows some features of epimorphic regeneration

Lacticaseibacillus rhamnosus GG enhances fin regeneration under oxytetracycline exposure via activating Wnt signaling and modulating gut microbiota

Zebrafish possesses robust caudal fin regeneration which depends on multiple factors to maintain body integrity. However, it is uncertain whether the caudal fin regeneration is related to gut microbiota. Here, we investigated the effect of Lacticaseibacillus rhamnosus GG (LGG) on the regeneration of caudal fin under oxytetracycline (OTC) exposure. The results demonstrated that 1000 μg/L OTC exposure for 4 days decreased reactive oxygen species (ROS) production at 1 and 3 h post amputation (hpa), increased neutrophil recruitment at 6 hpa, enhanced the number of apoptotic cells at 1, 3, 6 and 12 hpa and inhibited Wnt signaling pathway at 48 hpa in wound site. Furthermore, OTC exposure caused dysbacteriosis by elevating level of Proteobacteria and decreasing the abundance of Firmicutes, particularly Lacticaseibacillus, thereby negatively impacting wound healing and repair. Additionally, the administration of 106 CFU/mL of LGG for 48 h could improve intestinal environment through increasing the colonization rate of LGG in OTC-treated larvae intestines. The regenerative process restored by LGG was accompanied with increased ROS production at 1, 3 and 6 hpa, inhibited neutrophil recruitment at 6 hpa, decreased the number of apoptotic cells at 1 hpa, and activated Wnt signaling pathway at 48 hpa in OTC-treated fish. LGG is a promising bacterium for restoring fin regeneration and provides new insights regarding the correlation among the gut microbiota and fin regeneration.

Cartilage regeneration in zebrafish depends on Nrg1/ErbB signaling pathway

Objective: Cartilage, as the majority of adult mammalian tissues, has limited regeneration capacity. Cartilage degradation consecutive to joint injury or aging then leads to irreversible joint damage and diseases. In contrast, several vertebrate species such as the zebrafish have the remarkable capacity to spontaneously regenerate skeletal structures after severe injuries. The objective of our study was to test the regenerative capacity of Meckel's cartilage (MC) upon mechanical injury in zebrafish and to identify the mechanisms underlying this process. Methods and Results: Cartilage regenerative capacity in zebrafish larvae was investigated after mechanical injuries of the lower jaw MC in TgBAC(col2a1a:mCherry), to visualize the loss and recovery of cartilage. Confocal analysis revealed the formation of new chondrocytes and complete regeneration of MC at 14 days post-injury (dpi) via chondrocyte cell cycle re-entry and proliferation of pre-existing MC chondrocytes near the wound. Through expression analyses, we showed an increase of nrg1 expression in the regenerating lower jaw, which also expresses Nrg1 receptors, ErbB3 and ErbB2. Pharmacological inhibition of the ErbB pathway and specific knockdown of Nrg1 affected MC regeneration indicating the pivotal role of this pathway for cartilage regeneration. Finally, addition of exogenous NRG1 in an in vitro model of osteoarthritic (OA)-like chondrocytes induced by IL1β suggests that Nrg1/ErbB pathway is functional in mammalian chondrocytes and alleviates the increased expression of catabolic markers characteristic of OA-like chondrocytes. Conclusion: Our results show that the Nrg1/ErbB pathway is required for spontaneous cartilage regeneration in zebrafish and is of interest to design new therapeutic approaches to promote cartilage regeneration in mammals.

Genetically engineered zebrafish as models of skeletal development and regeneration

Zebrafish (Danio rerio) are aquatic vertebrates with significant homology to their terrestrial counterparts. While zebrafish have a centuries-long track record in developmental and regenerative biology, their utility has grown exponentially with the onset of modern genetics. This is exemplified in studies focused on skeletal development and repair. Herein, the numerous contributions of zebrafish to our understanding of the basic science of cartilage, bone, tendon/ligament, and other skeletal tissues are described, with a particular focus on applications to development and regeneration. We summarize the genetic strengths that have made the zebrafish a powerful model to understand skeletal biology. We also highlight the large body of existing tools and techniques available to understand skeletal development and repair in the zebrafish and introduce emerging methods that will aid in novel discoveries in skeletal biology. Finally, we review the unique contributions of zebrafish to our understanding of regeneration and highlight diverse routes of repair in different contexts of injury. We conclude that zebrafish will continue to fill a niche of increasing breadth and depth in the study of basic cellular mechanisms of skeletal biology.

Screening of Mineralogenic and Osteogenic Compounds in Zebrafish—Tools to Improve Assay Throughput and Data Accuracy

Bone disorders affect millions of people worldwide and treatments currently available often produce undesirable secondary effects or have limited efficacy. It is therefore of the utmost interest for patients to develop more efficient drugs with reduced off-target activities. In the long process of drug development, screening and preclinical validation have recently gained momentum with the increased use of zebrafish as a model organism to study pathological processes related to human bone disorders, and the development of zebrafish high-throughput screening assays to identify bone anabolic compounds. In this review, we provided a comprehensive overview of the literature on zebrafish bone-related assays and evaluated their performance towards an integration into screening pipelines for the discovery of mineralogenic/osteogenic compounds. Tools available to standardize fish housing and feeding procedures, synchronize embryo production, and automatize specimen sorting and image acquisition/analysis toward faster and more accurate screening outputs were also presented.

Application of zebrafish in the study of craniomaxillofacial developmental anomalies

Craniomaxillofacial developmental anomalies are one of the most prevalent congenital defects worldwide and could result from any disruption of normal development processes, which is generally influenced by interactions between genes and the environment. Currently, with the advances in genetic screening strategies, an increasing number of novel variants and their roles in orofacial diseases have been explored. Zebrafish is recognized as a powerful animal model, and its homologous genes and similar oral structure and development process provide an ideal platform for studying the contributions of genetic and environmental factors to human craniofacial malformations. Here, we reviewed zebrafish models for the study of craniomaxillofacial developmental anomalies, such as human nonsyndromic cleft lip with or without an affected palate and jaw and tooth developmental anomalies. Due to its potential for gene expression and regulation research, zebrafish may provide new perspectives for understanding craniomaxillofacial diseaseand its treatment.

Laser-mediated osteoblast ablation triggers a pro-osteogenic inflammatory response regulated by reactive oxygen species and glucocorticoid signaling in zebrafish

In zebrafish, transgenic labeling approaches, robust regenerative responses and excellent in vivo imaging conditions enable precise characterization of immune cell behavior in response to injury. Here, we monitored osteoblast-immune cell interactions in bone, a tissue which is particularly difficult to in vivo image in tetrapod species. Ablation of individual osteoblasts leads to recruitment of neutrophils and macrophages in varying numbers, depending on the extent of the initial insult, and initiates generation of cathepsin K+ osteoclasts from macrophages. Osteoblast ablation triggers the production of pro-inflammatory cytokines and reactive oxygen species, which are needed for successful macrophage recruitment. Excess glucocorticoid signaling as it occurs during the stress response inhibits macrophage recruitment, maximum speed and changes the macrophage phenotype. Although osteoblast loss is compensated for within a day by contribution of committed osteoblasts, macrophages continue to populate the region. Their presence is required for osteoblasts to fill the lesion site. Our model enables visualization of bone repair after microlesions at single-cell resolution and demonstrates a pro-osteogenic function of tissue-resident macrophages in non-mammalian vertebrates.

Regenerative Polarity of the Fin Ray in Zebrafish Caudal Fin and Related Tissue Formation on the Cut Surface

Zebrafish caudal fin rays are used as a model system for regeneration because of their high regenerative ability, but studies on the regeneration polarity of the fin ray are limited. To investigate this regeneration polarity, we made a hole to excise part of the fin ray and analyzed the regeneration process. We confirmed that the fin rays always regenerated from the proximal margin toward the distal margin, as previously reported; however, regeneration-related genes were expressed at both the proximal and distal edges of the hole in the early stage of regeneration, suggesting that the regenerative response also occurs at the distal edge. One difference between the proximal and distal margins is a sheet-like tissue that is formed on the apical side of the regenerated tissue at the proximal margin. This sheet-like tissue was not observed at the distal edge. To investigate whether the distal margin was also capable of forming this sheet-like tissue and subsequent regeneration, we kept the distal margin separated from the proximal margin by manipulation. Consequently, the sheet-like tissue was formed at the distal margin and regeneration of the fin ray was also induced. The regenerated fin rays from the distal margin protruded laterally from the caudal fin and then bent distally, and their ends showed the same characteristics as those of the normal fin rays. These results suggest that fin rays have an ability to regenerate in both directions; however, under normal conditions, regeneration is restricted to the proximal margin because the sheet-like tissue is preferentially formed on the apical side of the regenerating tissue from the proximal margin.

Musculoskeletal regeneration: A zebrafish perspective

Musculoskeletal injuries are common in humans. The cascade of cellular and molecular events following such injuries results either in healing with functional recovery or scar formation. While fibrotic scar tissue serves to bridge between injured planes, it undermines functional integrity. Hence, faithful regeneration is the most desired outcome; however, the potential to regenerate is limited in humans. In contrast, various non-mammalian vertebrates have fascinating capabilities of regenerating even an entire appendage following amputation. Among them, zebrafish is an important and accessible laboratory model organism, sharing striking similarities with mammalian embryonic musculoskeletal development. Moreover, clinically relevant muscle and skeletal injury zebrafish models recapitulate mammalian regeneration. Upon muscle injury, quiescent stem cells - known as satellite cells - become activated, proliferate, differentiate and fuse to form new myofibres, while bone fracture results in a phased response involving hematoma formation, inflammation, fibrocartilaginous callus formation, bony callus formation and remodelling. These models are well suited to testing gene- or pharmaco-therapy for the benefit of conditions like muscle tears and fractures. Insights from further studies on whole body part regeneration, a hallmark of the zebrafish model, have the potential to complement regenerative strategies to achieve faster and desired healing following injuries without any scar formation and, in the longer run, drive progress towards the realisation of large-scale regeneration in mammals. Here, we provide an overview of the basic mechanisms of musculoskeletal regeneration, highlight the key features of zebrafish as a regenerative model and outline the relevant studies that have contributed to the advancement of this field.

Skeletal Biology and Disease Modeling in Zebrafish

Zebrafish are teleosts (bony fish) that share with mammals a common ancestor belonging to the phylum Osteichthyes, from which their endoskeletal systems have been inherited. Indeed, teleosts and mammals have numerous genetically conserved features in terms of skeletal elements, ossification mechanisms, and bone matrix components in common. Yet differences related to bone morphology and function need to be considered when investigating zebrafish in skeletal research. In this review, we focus on zebrafish skeletal architecture with emphasis on the morphology of the vertebral column and associated anatomical structures. We provide an overview of the different ossification types and osseous cells in zebrafish and describe bone matrix composition at the microscopic tissue level with a focus on assessing mineralization. Processes of bone formation also strongly depend on loading in zebrafish, as we elaborate here. Furthermore, we illustrate the high regenerative capacity of zebrafish bones and present some of the technological advantages of using zebrafish as a model. We highlight zebrafish axial and fin skeleton patterning mechanisms, metabolic bone disease such as after immunosuppressive glucocorticoid treatment, as well as osteogenesis imperfecta (OI) and osteopetrosis research in zebrafish. We conclude with a view of why larval zebrafish xenografts are a powerful tool to study bone metastasis. © 2021 American Society for Bone and Mineral Research (ASBMR).

Visualization of extracellular vesicles in the regenerating caudal fin blastema of zebrafish using in vivo electroporation

Zebrafish have high regenerative ability in several organs including the fin. Although various mechanisms underlying fin regeneration have been revealed, some mechanisms remain to be elucidated. Recently, extracellular vesicles (EVs) have been the focus of research with regard to their role in cell-to-cell communication. It has been suggested that cells in regenerating tissues communicate using EVs. In this study, we examined the involvement of EVs in the caudal fin regeneration of zebrafish using an in vivo electroporation method. The process of regeneration appeared normal after in vivo electroporation, and the transferred plasmid showed mosaic expression in the blastema. We took advantage of this mosaic expression to observe the distribution of exosomal markers in the blastema. We transferred exosomal markers by in vivo electroporation and identified EVs in the regenerating caudal fin. The results suggest that blastemal cells communicate with other cells via EVs during caudal fin regeneration.

Zebrafish can regenerate endoskeleton in larval pectoral fin but the regenerative ability declines

We show for the first time endoskeletal regeneration in the developing pectoral fin of zebrafish. The developing pectoral fin contains an aggregation plate of differentiated chondrocytes (endochondral disc; primordium for endoskeletal components, proximal radials). The endochondral disc can be regenerated after amputation in the middle of the disc. The regenerated disc sufficiently forms endoskeletal patterns. Early in the process of regenerating the endochondral disc, epithelium with apical ectodermal ridge (AER) marker expression rapidly covers the amputation plane, and mesenchymal cells start to actively proliferate. Taken together with re-expression of a blastema marker gene, msxb, and other developmental genes, it is likely that regeneration of the endochondral disc recaptures fin development as epimorphic limb regeneration does. The ability of endoskeletal regeneration declines during larval growth, and adult zebrafish eventually lose the ability to regenerate endoskeletal components such that amputated endoskeletons become enlarged. Endoskeletal regeneration in the zebrafish pectoral fin will serve as a new model system for successful appendage regeneration in mammals.